docs: add documentation for generate CLI command (#1763)

Author: ldetmer

doc/cli-commands.md

@@ -0,0 +1,34 @@
+# Generate Command
+
+The `generate` command is used to generate client library code for a repository.
+
+## Usage
+
+```bash
+librarian generate [flags]
+```
+
+## Flags
+
+| Flag           | Type    | Required | Description |
+|----------------|---------|----------|-------------|
+| `-api`         | string  | No (Yes for onboarding) | Path to the API to be configured (e.g., `google/cloud/functions/v2`). |
+| `-api-source`  | string  | No       | Location of the API repository. If undefined, googleapis will be cloned to the output. |
+| `-build`       | bool    | No       | Whether to build the generated code after generation. |
+| `-host-mount`  | string  | No       | A mount point from Docker host and within Docker. Format: `{host-dir}:{local-dir}`. |
+| `-image`       | string  | No       | Language specific container image. Defaults to the image in the pipeline state. |
+| `-library`     | string  | No (Yes for onboarding) | The ID of a single library to update or onboard.  If updating this should match the library ID in the state.yaml file. |
+| `-repo`        | string  | No       | Code repository for the generated code. Can be a remote URL (e.g., `https://github.com/{owner}/{repo}`) or a local path. If not specified, will try to detect the current working directory as a language repository. |
+| `-output`      | string  | No       | Working directory root. If not specified, a working directory will be created in `/tmp`. |
+| `-push`        | bool    | No       | Whether to push the generated code and create a pull request. |
+
+## Example
+
+```bash
+librarian generate -repo=https://github.com/googleapis/your-repo -library=your-ilbrary-id -build -push
+```
+
+## Behavior
+
+- **Onboarding a new library:** Specify both `-api` and `-library` to configure and generate a new library.
+- **Regenerating an existing library:** Specify `-library` to regenerate a single library. If this flag is not provided, all libraries in `.librarian/state.yaml` are regenerated.